1. Field of Invention
The present invention relates to an engine knocking meter for automatically measuring knocking which occurs during abnormal combustion condition of an engine.
2. Prior Art
It is important to determine knocking which occurs during abnormal combustion condition of an engine in measuring performance of the engine such as an ignition advance angle of the engine for improving the performance of the engine. As the trend of lead-free gasoline is enhanced, the measurement of knocking is essential. In the past, the measurement of knocking has been relied on an experienced person's listening to knocking sound by his ears. Although this method is superior in that the knocking can be determined without requiring any modification of the engine, it has disadvantages that slight change in knocking sound cannot be determined and the determination of the knocking is difficult under a big noise condition that occurs during high speed rotation of the engine. Methods for automatically measuring the knocking have also been proposed.
One method thereof uses an acceleration detection system. In this method, vibration conveyed through a rigid body of the engine is sensed by an acceleration meter to measure abnormal vibration due to the knocking. Although this method is somewhat superior to the above method of determining the knocking by human ears in that the former can automatically determine the knocking, it still has a disadvantage that a detection sensitivity greatly changes depending on a position at which the acceleration meter is mounted. Furthermore, under the high speed rotation condition of the engine, it is very difficult to distinguish the vibration of the engine per se from the vibration due to the knocking.
A second method applies a system for detecting a pressure within a combustion chamber. In this method, an engine pressure meter using a semiconductor strain gauge or the like is used to detect abnormal change in pressure due to the knocking which is superimposed on a combustion pressure signal of a cylinder. Although this method is advantageous in that the knocking can be precisely determined, it has a disadvantage that it is difficult to mount the engine pressure meter because of its characteristic and structure and hence the operability is poor. Furthermore, this method is expensive.
A third method applies a pressure sensing plug system. In this method, a washer portion of a spark plug is made of a piezoelectric transducer of ceramics so that a pressure applied to the spark plug is changed in accordance with the change of pressure within the cylinder, and the change in the pressure applied to the plug is detected as an electrical signal by piezoelectric effect of the piezoelectric transducer of ceramics. This method is relatively effective because no modification of the engine is required since the piezoelectric element is of small size and it can be readily mounted or detached and it is cheap. However, this method has a drawback that undesired noises such as noises due to engine vibration or ignition noises are introduced in the knocking signal because the piezoelectric element in nature has a very high output impedance of the order of 10.degree. .OMEGA. or higher and hence the ignition noises are greatly introduced and the vibration of the engine is readily picked up. It has a disadvantage of small S/N ratio particularly when the engine is rotating at a high speed.
All of the prior art methods described above have advantages as well as disadvantages and they are not satisfactory in achieving ideal determination of the knocking.
FIGS. 1A and 1B show waveforms of electrical signals representative of pressures in the cylinder detected by the pressure sensing spark plug. FIG. 1A shows a combustion pressure waveform detected under normal combustion condition while FIG. 1B shows a combustion pressure waveform detected under abnormal combustion condition where knocking is occurring in the engine. In FIGS. 1A and 1B, reference character A denotes a signal indicative of a pressure waveform in the cylinder, reference character B denotes a signal indicative of ignition noise waveform, reference character C denotes a signal indicative of abnormal pressure waveform due to the knocking, reference character D denotes a signal indicative of noise waveform due to the operation of a contact point, and reference character E denotes a signal of vibration noise waveform caused by mechanical vibration noise of the engine.
As seen from FIGS. 1A and 1B, the pressure detecting signal includes complex and various noises, and an S/N ratio between the engine knocking signal and other noises is very low.